1. Field of the Invention
The present invention relates to a polishing agent for use in polishing or lapping of workpieces, and more particularly to a polishing agent used preferably when lapping silicon wafers, etc. (note that the polishing agent of the invention is a so-called loose abrasive and that it is not intended to be limited by its name whether it is called “polishing agent” or “lapping agent”).
2. Description of the Related Arts
Conventionally, in the manufacture of silicon wafers used as substrates of integrated circuits such as ICs and LSIs, and discrete semiconductor devices such as transistors and diodes, it is general, first, to grow a silicon ingot in Czochralski method (CZ method), floating zone melting method (FZ method) or the like, to slice out wafers from the grown ingot with an inner diameter slicer or a wire saw, and to subject the obtained wafers to a chamfering process for preventing cracking of a peripheral edge portion of the wafers, a lapping process for improving the flatness of the wafers, an etching process for removing mechanical damages and contaminants, and a mirror-polishing process for mirror-polishing the one main surface or both surfaces of each wafer in the order.
In these silicon wafer manufacturing processes, the lapping process described above is a rough polishing process conducted prior to the mirror-polishing process. As a common lapping method, for example, a silicon wafer is held between an upper turn table and a lower turn table of a lapping apparatus and the wafer can be subjected to lapping by rotating the upper and the lower turn tables while being supplied with loose abrasive grains in which a polishing agent is dispersed in a solvent such as water. By performing this lapping process, a mechanically damaged layer created when the wafer is sliced out can be removed as well as the flatness of the wafer can be improved.
As a polishing agent used in the lapping process, alumina-based fine lapping materials are currently used mainly. Among them, as the polishing agent for lapping silicon wafers, FO abrasive grain (manufactured by Fujimi Incorporated) that is a composition of alumina and zircon sand is used and it accounts for almost 100 percent of the total use in Japan. Especially, those having a grain size of, for example, FO abrasive grain #1200 (the maximum diameter; 23.0 μm or smaller, the average grain diameter: 7.1 μm) are often used since they well match the surface precision of silicon wafers (see “Precision Process Technology of Crystalline Material for Electronics”, Science Forum Inc., Edited by Masahisa Matsunaga et al., pp. 257–261)).
In the case for a polishing agent of such composition of alumina and zircon sand, a workpiece is ground basically by the action of alumina contained in the polishing agent, and occurrence of scratches caused by the polishing can be suppressed as well as the surface of the workpiece can be flattened by the action of zircon sand. Therefore, by utilizing these actions of alumina and zircon, the workpiece can be subjected to the lapping.
However, with the recent finer processes and higher integration degree of semiconductor devices, efforts have been made for improving the performance of polishing agents by advancing still currently improvement and development of polishing agents such that higher-quality wafers with higher flatness can be obtained efficiently (see, e.g., Japanese Patent Application Laid-open (Kokai) No. 8-17770).
In general, silicon wafers having been lapped (hereinafter, referred to simply as “lapped wafers”) are often evaluated by qualities such as their flatness, surface roughness, scratches, etc. Therefore, such a polishing agent is demanded as the one which can further improve these wafer qualities as well as the one which is excellent in polishing efficiency in terms of improvement of productivity, i.e., the one which has as high polishing rate of wafers as possible.
However, in the case of a polishing agent as described above comprising a composition of alumina and zircon sand, the polishing ability that alumina originally has is lost because zircon sand is contained in the agent. Therefore, the agent has a disadvantage that it has a lower polishing rate compared to a polishing agent comprising only alumina. On the other hand, when the amount of alumina contained is increased to utilize the polishing ability of alumina, the flattening ability for flattening a workpiece, that the polishing agent has is weakened because the amount of zircon sand contained is decreased, and scratches are liable to be generated on the workpiece. For this reason, there is a problem that the quality of a workpiece after lapping cannot be improved any more.